Exchange 2010 SP2 Rollup 5v2 fails to install (KB2785908):
http://social.technet.microsoft.com/Forums/en-US/exchange2010/thread/7932aecb-8fa9-4a18-a605-ae6fd1020064
…and confirmed.
You’ll get a 80070643 error installing KB2785908 if KB2506143 is installed or comes down with the update rollup.
Remove 2506143 (WMF 3.0) from that server.